The present invention relates to a multiband antenna switch which can be used for switching between a transmitting branch and a receiving branch of a multiband mobile radio telephone.
In recent years, mobile radio networks of different standards have been developed which operate in different frequency bands. For example, the mobile radio network of the GSM900 standard operates within a range of 900 MHz, the mobile radio network of the GSM1800 standard operates in a range of 1 800 MHz and the mobile radio network of the PCS1900 standard operates in a range of 1 900 MHz.
Accordingly, it is desirable to create mobile radio telephones or similar devices which can be operated in a number of different frequency bands, that is to say which are capable of operating at different standards of mobile radio networks.
Furthermore, it is required in mobile radio telephones for, for example, the GSM900 and GSM1800 standards, to switch the transmitting and receiving branches of the mobile radio telephones in accordance with the TDMA concept for GSM mobile radio telephones. This means that it is necessary that the transmitting and receiving branches are active in a manner so as to be differentiated from one another in time.
FIG. 5 shows a block diagram of a dual-band antenna switch of the prior art which is used for switching between the transmitting and receiving branches of a mobile radio telephone operating in the GSM (900 MHz) and PCN (1 800 MHz) standards. This antenna switch consists of a diplexer 11 separating/combining the frequency bands of the GSM and PCN standards, a first change-over switch 12, a second change-over switch 13, a first low-pass filter 14 and a second low-pass filter 15. The antenna switch also exhibits a GSM transmit terminal GSM-TX, a GSM receive terminal GSM-RX, a PCN transmit terminal PCN-TX, a PCN receive terminal PCN-RX, an antenna terminal and four control terminals VC1 to VC4.
The operation of this multiband antenna switch is as follows:
To the four control terminals VC1 to VC4, respective control signals are applied which are used for setting the first and second change-over switch 12 and 13, respectively, to a transmit or receive mode. The relation between the various signals applied to the control terminals and the respective mode is shown in the table below.
The table shows that, with a high signal level at control terminal VC1, the second change-over switch 13 is connected to the GSM transmit terminal GSM-TX via the second low-pass filter 15, that with a high signal level at the control terminal VC2, the second change-over switch 13 is connected to the GSM receive terminal GSM-RX, that with a high signal level at the control terminal VC4, the first change-over switch 12 is connected to the PCN transmit terminal PCN-TX via the first low-pass filter 15 and that with a high signal level at the control terminal VC3, the first change-over switch 12 is connected to the PCN receive terminal PCN-RX.
In the GSM transmit mode (VC1 at high signal level), a GSM transmit signal is output by the GSM transmit terminal GSM-TX via the second low-pass filter 15, the second change-over switch 13 and the diplexer 11 to an antenna connected to the antenna terminal and radiated by this antenna.
In GSM receive mode (VC2 at high signal level), a GSM receive signal is input into the GSM receive terminal GSM-RX from an antenna connected to the antenna terminal, via a diplexer 11 and the second change-over switch 13.
In PCN transmit mode (VC4 at high signal level), a PCN transmit signal is output by the PCN transmit terminal PCN-TX via the first low-pass filter 14, the first change-over switch 12 and the diplexer 11 to an antenna connected to the antenna terminal and radiated by this antenna.
In PCN receive mode (VC3 at high signal level), a PCN receive signal is input into the PCN receive terminal PCN-RX from an antenna connected to the antenna terminal, via the diplexer 11 and the first change-over switch 12.
It should be noted that the multiband antenna switch described above is a GaAs-integrated switch which contains four field effect transistors, in each case one as two-terminal switch, and one diplexer.
However, the antenna switch described above has the following disadvantages. The diplexer 11 is connected directly to the antenna terminal and then there are the first and second change-over switches 12 and 13, respectively, in the PCN branch and in the GSM branch, respectively. This requires two change-over switches which must also be additionally activated with a total of four different control signals. However, providing such change-over switches is complex and expensive since the change-over switches must be suitable for the frequencies of 900 and 1 800 MHz used. In addition, the four necessary control signals require an elaborate external logic for controlling the change-over switches. Finally, the antenna switch has large external dimensions.
The present invention has been created with regard to the above problems and, accordingly, its object consists in creating a multiband antenna switch which is inexpensive and small and which also needs to be activated with only a single control signal.
According to the invention, this object is achieved by means of the measures specified in claim 1.
More precisely, a multiband antenna switch according to the invention exhibits a transmitting and a receiving branch which are arranged in parallel with one another and are connected to one another at a circuit node, a matching pad arranged in the receiving branch, a filtering device arranged between the circuit node and an antenna terminal, and a switching device which can be activated by means of a predetermined logic signal, in which the switching device, following an application of the predetermined logical signal, connects the transmitting branch to the antenna terminal and otherwise the transmitting branch is separated from the antenna terminal and the receiving branch is connected to the antenna terminal, the matching pad, in the transmit case, effects an open-circuit operation of the receiving branch at the circuit node with respect to at least one higher-frequency useful-frequency band and the matching pad and the filtering device are tuned to one another in such a manner that, in the transmit case, a non-resistive component of an input impedance of the matching pad is essentially compensated for at the circuit node with respect to at least one lower-frequency useful-frequency band.
This makes it possible to realize a multiband antenna switch which has only a single switch and which can also be activated by means of a single control signal.
The transmitting branch preferably has a transmit terminal to which a first diplexer combining the useful-frequency bands is connected and the receiving branch has a receive terminal to which a second diplexer separating the useful-frequency bands is connected.
Further advantages are achieved due to the presence of the two diplexers directly at the transmit and receive terminals of the transmitting and receiving branches, respectively. The diplexers have a high filtering effect so that, for example, harmonics can be filtered out. In addition, the structure as a whole is inexpensive due to the low price of the diplexers and can also be implemented with small dimensions.
Furthermore, the switching device preferably consists of a first diode connected in series in the transmitting branch and a second diode connected to ground in parallel with the receiving branch at a terminal of the matching pad opposite to the circuit node in the receiving branch.
Due to the fact that the switching device only consists of the two diodes, the switching device as a whole is inexpensive and can be produced with small dimensions.
Further advantageous embodiments of the present invention are the subject matter of the subclaims.